Note: Descriptions are shown in the official language in which they were submitted.
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CONVECTION COMBUSTION OVEN
[00001]
BACKGROUND OF THE INVENTION
[00002] The present invention relates toward an inventive oven
for curing coatings applied to an object. More specifically, the present
invention relates to a convection combustion oven having a simplified
design for curing coatings applied to an object.
[00003] Various types of ovens are used to cure coatings, such
as, for example, paint and sealers, that are applied to articles in a
production setting. One example is decorative and protective paint that is
applied to automotive vehicle bodies in a high volume paint shop known
to process vehicle bodies at rates exceeding one per minute.
[00004] A typical oven uses combustion fuel to provide the
necessary amount of heat to cure paint applied to a vehicle body.
Generally two types of ovens are presently used, a convection oven and a
radiant heat oven. Occasionally, a combination of convection and radiant
heat is used in a single oven to meet paint curing specifications. A
convection heat oven makes use of a heat source such as natural gas
flame that heats pressurized air prior to delivering the heated air to an
oven housing. A first type of convection heating applies combustion heat
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directly to pressurized air prior to delivery to the oven housing mixing
combustion gases with the pressurized air. A second type of convection
heating uses an indirect heating process where combustion heat is
directed into a heat exchanger that heats the pressurized air without
mixing the combustion gases with the pressurized air.
[00005] An alternative source of heat is provided inside the
oven housing by a radiant heater that transfers heat to the vehicle body
by way of proximity to the vehicle body. As known to those of skill in the
art, a radiant heater is generally a metal panel that is heated by
circulating hot air into a space located behind a radiator.
[00006] The conventional convection and radiant ovens have
proven to be exceedingly expensive to construct and do not provide
energy efficiencies desirable in today's high-cost energy market. A
conventional oven design is generally shown at 10 in Figure 1. The
conventional oven assembly 10 generally includes two main components,
a heater box 12 and an oven housing 14. The heater box 12 is generally
spaced from the oven housing 14 and includes components (not shown) to
provide heat and pressurized air to the oven housing 14 through hot air
duct 16. The heater box 12 includes a return duct that draws a significant
portion of air from the interior of the oven housing 14 for recirculation
through the oven housing 14. Up to ninety percent of the air passing
through the heater box 12 is derived from the interior of the oven housing
14 through return duct 16. Generally, only ten percent of the air
delivered to the oven housing 14 through hot air duct 16 is fresh air
drawn from outside the oven housing 14. Hot air is directed through hot
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air headers 20 toward the vehicle body through nozzles 22 to optimize a
uniform heat transfer to cure the coating applied to the vehicle body.
Generally, the vehicle body is heated to about 275-3400 F at a
predetermined time to adequately cure the applied coating. Some
coatings, such as electrodeposition primers, require temperatures at the
higher end of this range. As is known to those of skill in the art, more
heat must be directed toward heavy metal areas of the vehicle body to
derive the desired baking temperature.
[00007] A typical oven zone of about eighty feet in length of a
conventional oven requires an actual air volume of about 30,000 cfm
when using a heater box. This high air volume is required to transfer the
necessary heat to the vehicle body to cure the applied coating. The air
temperature at the nozzle 22 in a conventional oven is generally 4440 F
requiring an air velocity at the nozzle 22 of 4,930 fpm to transfer the
desired amount of heat energy. The operating parameter set forth above
generally provides 1,595,000 BTU/hr at a momentum of 4.9 x 106 ft-
Ib/sec2. Because hot air is recirculated by the fan located in the heater
box 12, and because the recirculated air is often reheated prior to being
pressurized by the fan, the fan requires an overlying robust design adding
to operation and installation costs.
[00008] The volumes and flow rates presently used in
conventional ovens require heavy duty fans and heater systems that are
not believed necessary to obtain the required heat transfer. This is in part
due to the recirculation of hot air through the fan and back into the oven
housing 12. Furthermore, due to the recirculation, a substantial amount
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of insulation 24 is required around the heater box 12 and the hot air duct
16 to reduce heat loss and protect workers from physical contact.
Therefore, it would be desirable to design a simplified oven assembly that
does not require extensive insulation and complex apparatus associated
with conventional heater boxes.
SUMMARY OF THE INVENTION
[00009] The present invention discloses an oven assembly for
curing a coating applied to an article being conveyed through the oven
assembly. A transporter extends through an oven housing for conveying
the article through the oven assembly. A fan provides pressurized air into
the oven housing drawn substantially from outside the oven housing. A
duct includes a first element extending into the oven housing and a
second element interconnected with the fan for transporting pressurized
air from the fan into the oven housing. A burner is disposed generally
between the first element and the second element for heating the
pressurized air being transported into the oven housing. The first element
defines a plurality of air outlets spaced throughout the oven housing for
directing heated air toward the article. The first element is substantially
insulated inside the oven housing reducing the escape of heat generated
by the burner from the duct except through the air outlet. The burner
heats the pressurized air being directed into the oven housing to a
temperature of about three times the curing temperature of the coating
that is applied to the article.
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[00010] The inventive oven assembly solves the problems
associated with the prior art, or conventional oven assembly. Particularly,
the
size of the ventilator or fan used to provide pressurized air to the oven
housing for transferring heat to the article being baked is significantly
reduced
for two reasons. First, the fan primarily draws ambient temperature air as the
present design does not circulate heated air back into the oven housing and,
therefore, does not need to be heat resistant. Furthermore, the heater or
burner used to heat the ambient temperature air prior to the introduction to
the first element of the duct is configured to heat the air to about two to
four
times the curing temperature of the coating applied to the vehicle body
adjacent the oven housing. This temperature air, when introduced to the oven
interior at a high nozzle velocity, reduces the air volume of a conventional
80
foot long oven zone from about 30,000 acfm to about 2,000 scfm. At this
combination of air volume, air temperature, and air velocity, a substantially
similar amount of BTUs per hour is delivered to the oven as a conventional
oven while using less energy to drive the ventilator and having a
significantly
simplified ventilation and heating apparatus. Specifically, the complex heater
box presently used in conventional ovens is no longer necessary and is,
therefore, completely eliminated substantially simplifying the construction
and
design of a production oven.
DETAILED DESCRIPTION OF THE INVENTION
[00011] Referring to Figure 2, an inventive oven assembly is
generally shown at 30. The oven assembly includes an oven housing 32
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through which an article such as, for example, a vehicle body 34 is conveyed
on a transporter 36. The transporter 36, as is known to those of skill in the
art, is generally designed as a conveyor that conveys a carrier 38 upon which
the vehicle body 34 is secured.
[00012] In a production paint shop, a coating is applied to the
vehicle body 34 providing decorative and protective paint finish to the
vehicle
body 34. Different coatings have different baking or curing requirements that,
along with vehicle body type and production volume, dictate the length and
thermal requirements of the inventive oven assembly 30. For example,
electrodeposition primers typically cure at about 3400 F for about twenty
minutes and decorative top coat and clear coats cure at about 2850 F also for
about twenty minutes. For simplicity, the explanation of the inventive
concepts of the present oven assembly 30 will assume a typical eighty foot
long oven zone requiring a delivery of heat of about 1,595,000 British thermal
minutes per hour (BTU/hr).
[00013] Pressurized air is delivered into the oven housing 32
through a duct 40 by a ventilator 42. Preferably, the ventilator 42 is a
conventional fan capable of providing the transfer of ambient air at a volume
of about 2,000 scfm. The duct 40 includes a first element 44 generally
extending inside the oven housing 32 and a second element 46 generally
extending from the ventilator 42 to the first element 44. A heater 48 is
disposed between the first element 44 and the second element 46 to provide
heat to the pressurized air passing through the duct 40 as delivered by the
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ventilator 42. Preferably, the heater 48 is a gas fired burner sized to
provide
the desired amount of heat to the pressurized air passing through the duct 40
to adequately cure the coating applied to the vehicle body 34. However, it
should be understood by those of skill in the art, that alternative heaters
may
also be used to provide heat to the pressurized air as set forth above.
[00014] As will be explained further below, the heater increases the
temperature of the pressurized air to about 1,1000 F or hotter. One range
contemplated is between about 7000 and 1,1000 F The desired temperature is
selected to be between about two and four times the curing temperature of
the coating as will be explained further below. The heater is located,
preferably, adjacent to or nearly adjacent to the oven housing 32 so that the
heated, pressurized air travels only through the interior of the oven housing
32. This reduces the need to insulate the duct 40, or more specifically, the
second element 46 of the duct 40 further reducing assembly costs. However,
insulation 50 covers the first element 44 of the duct 40 inside the oven
housing 32 to prevent the escape of heat through the first element 44 into the
oven housing 32 except where desired.
[00015] The oven assembly 30 represented in Figure 2 shows two
heaters 48 positioned on opposing sides of the oven housing 32, each
providing heat to opposing first elements 44. Therefore, the first element 44
of the duct 40 is disposed on opposing sides of the vehicle body 34 being
transported through the oven housing 32. However, it should be understood
that a single heater 48 is contemplated to provide heat to each of the
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opposing first elements 44 of the duct 40 by locating the heater 48 generally
midway between each of the opposing first elements 44.
[00016] Each first element 44 defines an upper header 52 and a
lower header 54 that extend in a generally horizontal direction. Nozzles 56
are spaced along each of the upper header 52 and lower header 54 through
which pressurized, heated air is projected toward predetermined locations on
the vehicle body 34. Figure 3 best represents the spaced locations of the
nozzles 56 on the upper header 52 and lower header 54, the configuration of
which will be explained further below. As best represented in Figure 3, a feed
header 58 extends between the heater 48 and the lower header 54 of the first
element 44. The feed header 58 serves as a mixer providing distance
between the first of the nozzles 56 and the heater 48 so that the combustion
gases produced by the heater 48 have ample time to mix with the pressurized
air provided by the ventilator 42. In this example, about eight feet in length
of the feed header 58 has proven to provide ample mixing time for the
combustion gases generated by the heater 48 in the pressurized air provided
by the ventilator 42 for an eighty foot oven zone. Different size oven
assemblies with different heat requirements may require different lengths of
feed headers 58. The first element 44 shown in Figure 3 shows in connection
serially, the feed header 58 with the lower header 54, which is connected to
the upper header 52 by a connection header 60. In this configuration, the
pressurized air travels a single path through the feed header 58 to the lower
header 54, through the connection header 60, terminating at a distal end 62
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of the upper header 52. It should be understood by those of skill in the art
that a heater 48 placed in a lower portion of the oven assembly 30 connects
first to the upper header 52 via feed header 58 reversing the direction of the
pressurized air through the first element 44.
[00017] Referring again to Figures 2 and 3, vertical temperature
probes 68 extend downwardly from the roof of the oven housing 32 to
measure the interior temperature of the oven housing 32. The vertical
temperature probes 68 communicate with a controller (not shown) that signals
the heaters 48 to adjust, when necessary, the interior temperature of the oven
housing 32. Horizontal temperature probes 70 are spaced below the vertical
temperature probes 68 and measure temperature in a similar manner as the
vertical temperature probes 68 the temperature of the oven in the lower
regions of the housing 32. Header temperature probes 72 extend into the
feed header 58 to measure the temperature of the pressurized air inside the
feed header 58 in a manner similar to that explained for the vertical
temperature probe 68 above. Each of the probes interact with the controller
to control the temperature of the interior of the oven housing 32. Additional
header temperature probes 72 may be spaced along the second element 46 if
necessary. For faster response, vertical or horizontal probes 68,70 may be
located directly in front of a nozzle 56, spaced from the nozzle 56 between
one
to three feet.
[00018] Referring to Figure 4, a cross-sectional view of one of the
upper header 52 and lower header 54 is shown. As set forth above, insulation
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50 surrounds a header wall 74 reducing the heat loss through the header wall
74 into the oven housing 32. The nozzles 56 are located inside the header
wall 74 and define a decreasing diameter from a distal end 76 toward a
terminal end 78 located generally adjacent the header wall 74. Therefore, the
nozzle 56 defines a generally concave, frustoconical shape so that the
pressurized air passing through the nozzle 56 accelerates due to decreasing
area upon exit from the first element 44. The shape of the nozzles 56 is best
represented in the perspective view shown in Figure 5A. Figure 5B shows an
alternative nozzle 57 having a swivel 80 that allows the alternative nozzle 57
to be articulated inside the first element 44 enabling the pressurized air to
be
directed to the predetermined location in a more accurate manner.
[00019] An alternative nozzle in the form of an eductor or venturi
nozzle is shown at 82 in Figure 6. The eductor 82 is shown in Figure 6 having
a mating surface 86 that is affixed to header wall 74 outside of the header
52,
54. The mating surface 86 defines a pressurized air inlet 88 that receives
pressurized air from one of the upper and lower header 52, 54. The
pressurized air passes through venturi chamber 90 and exits the eductor 82
through eductor nozzle 92 directing the pressurized air toward the
predetermined location of the vehicle body 34 as set forth above. Hot air is
drawn from the interior of the oven housing 32 through venturi inlet 94 and is
forced into the eductor nozzle 92 by the pressurized air passing through the
venturi chamber 90 via venturi effect as is known. This increases the
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volumetric flow of air toward the predetermined location of the vehicle body
34 further reducing the energy requirements of the ventilator 42.
[00020] A further embodiment nozzle is shown as an air amplifier
96 at Figure 7 where like numerals will be used with Figure 6 for simplicity.
The air amplifier 96 includes an air inlet 88 where pressurized air is forced
from one of the upper and lower headers 52, 54. The pressurized air passes
through the venturi chamber 90 and into the amplifier nozzle 92 and directs
the pressurized air toward a predetermined location of the vehicle body 34.
Heated air is drawn from the interior of the oven housing 32 through venturi
inlet 94 via the venturi effect causing an increase in the volumetric flow of
heated air directed toward the vehicle body 34 again reducing the energy
requirements of the ventilator 42.
[00021] The embodiments set forth above are desirable to heat
heavy metal areas of the vehicle body 34, which have higher heat
requirements than thin or sheet metal areas of the vehicle body 34. In these
embodiments, the eductor 84 and the air amplifier 96 are each directed at a
predetermined location of the vehicle body drawing heated air from inside the
oven housing 32 maximizing the amount of heat energy directed toward the
heavy metal area of the vehicle body 34. As explained above, pressurized air
passes through the header 52, 54 through air inlet 88 and into the venturi
chamber 90 prior to exiting through the nozzle 92. Hot air is drawn into
venturi inlet 94 via the venturi effect increasing the volumetric flow rate of
hot
air being directed toward the vehicle body 34.
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[00022] Table 1 shows the operational parameters of the inventive
oven assembly 30 that provides the benefits set forth above.
Convention New New New
al Oven Oven Oven Oven
Low High
Nominal Nominal Velocity Velocity
Design Design Case Case
1,595,21 1,595,21 1,595,21
Heat Delivered BTU/hr 1,595 217 7 7 7
ft
Momentum Ibm/sec
Delivered 2 1,365 1,365 836 1,643
Delivery Volume -
6 000
Actual acfm 30,000 6,000 6,000
Delivery Volume -
2 000
Standard scfm 17,584 2,000 2,000
Air Delivery
Temperature F 444 1,100 1,100 1,100
Number of Nozzles 72 72 44 97
Nozzle Diameter in 4.528 0.676 1.100 0.531
Air Velocity at
Nozzle fpm 3,727 32,000 20,000 40,000
Nozzle Velocity /
Volume 1/ft2 9 401 150 650
Nozzle Velocity /
Area 1/ft-sec 556 219,000 50,000 427,000
Air Volume / Oven
Length scfm/ft 220 25 25 25
Table 1
[00023] The data shown in Table 1 is based upon a typical 80 foot
long oven section (i.e., heat up zone) at a typical vehicle body 34 production
rate. In each example, the required heat delivery is about 1,595,000 BTU/hr.
The first column shows the various operating requirements to produce the
heat required in a conventional oven design and the following columns indicate
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the inventive oven nominal design, with a lower limit velocity and an upper
limit velocity establishing the general operating range.
[00024] Most notably, a significant reduction in the standard
delivery volume is realized in standard cubic feet per minute (ambient
temperature). Those of skill in the art will understand that delivery volume
in
a conventional oven is generally 30,000 acfm because hot air is recirculated
through the oven by the heater box 12 shown in Figure 1. Therefore the
reduction in delivery volume enabling a significant reduction in fan capacity
is
actually from 30,000 acfm to 2,000 scfm. To maintain the required heat
delivery at the reduced delivery volume, the air delivery temperature at the
nozzles 56 is increased to about 1,1000 F in the new oven design exceeding
the conventional air delivery temperature at a conventional nozzle 22 of about
4440 F. Additionally, the nozzle diameter is reduced from a conventional
diameter of about .38 feet to about .06 feet resulting in an increase in air
velocity at the nozzle from 3,727 fpm to about 32,000 fpm in the nominal
oven assembly 30. This provides a nominal nozzle velocity per area of nozzle
of about 219,000 ft-sec, much higher than the conventional nozzle velocity per
area of about 556 ft-sec. Therefore, the inventors have determined that a
momentum requirement for delivering heat energy remains constant when
pressurized air is delivered at up to three times higher than the curing
temperature of the coating applied to the vehicle body at higher air
velocities
and significantly lower delivery volume. Based upon studies, it is believed
that
temperatures of between two and four times the curing temperature in
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Fahrenheit degrees with a coating applied to the vehicle body is a preferred
operating range while still providing enough heat energy to cure or bake the
coating applied to the vehicle body. Furthermore, the ratio set forth above
makes use of an air velocity to air volume ratio at the nozzles 56 of between
about 150 and 650 to 1, with a nominal ratio of about 400 to 1. Furthermore,
the ratio of air velocity in feet per second to a nozzle area is determined to
be
between about 50,000 and 400,000 to 1, with a nominal velocity of about
220,000 to 1.
[00025] Further operating parameters proven to achieve desired
heat and momentum requirements include providing the volume of air to the
oven housing at less than about 25 scfm per foot of oven housing. An
alternate embodiment provides a volume of air to the oven housing of less
than about 50 scfm per foot of oven housing. A still further alternate
embodiment provides a volume of air to the oven housing at a rate of about
75 scfm per foot of oven housing. This is significantly less than a
conventional
oven design which requires about 220 scfm per foot oven length, requiring
higher energy usage than the inventive oven assembly 30.
[00026] An additional benefit of heating the pressurized air to about
1,1000 F is the ability to clean the oven 30 by combustion of coating
byproducts known to coat oven walls. This eliminates the need to manually
wash oven walls, which is labor intensive.
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[00027] The invention has been described in an illustrative manner,
and it is to be understood that the terminology which has been used is
intended to be in the nature of words of description rather than of
limitation.
[00028] Obviously, many modifications and variations of the
present invention are possible in light of the above teachings. It is,
therefore,
to be understood that within the scope of the appended claims, wherein
reference numerals are merely for convenience and are not to be in any way
limiting, the invention may be practiced otherwise than as specifically
described.
